A programmable read-only memory (PROM), a field programmable read-only memory (FPROM), an electrically erasable programmable read-only memory (EEPROM) and a one-time programmable non-volatile memory (OTP NVM) are forms of digital memory where the setting of each bit is locked by a fuse or an anti-fuse. These PROMs may be used to store programs permanently. One difference between a read-only memory (ROM) and a PROM is that with a PROM the programming is applied after the device is constructed.
PROMs are often manufactured blank and depending on the technology can be programmed on a wafer, during final test, or in a system. The availability of this technology allows companies to maintain a supply of blank PROMs in stock, and program them at the last minute to avoid a large volume commitment. These types of memories are frequently seen in video game consoles, mobile phones, radio-frequency identification tags, implantable medical devices, high-definition multimedia interfaces and in many other consumer and automotive electronic products.
An EEPROM can be erased and reprogrammed (written to) repeatedly through the application of higher than normal electrical voltage generated externally or internally in the case of modern EEPROMs. EEPROMs can be programmed and erased in the circuit in which they are found. EEPROMs allow multi-byte page operations. The number of times an EEPROM can be written is limited. In many applications, the limitation is approximately a million write operations. For this reason EEPROMs are typically used to provide configuration information rather than random information.
In some embodiments of an EEPROM, a FGMOSFET (floating gate metal oxide semiconductor field effect transistor) is used. The structure of a FGMOSFET) is similar to a conventional MOSFET (metal oxide semiconductor field effect transistor). The gate of the FGMOSFET is electrically isolated to create a floating node. One or more inputs are deposited above the floating gate and are electrically isolated from it. These inputs are only capacitively connected to the floating gate. Since the floating gate is surrounded by highly resistive material (insulators), the charge contained on the floating gate remains unchanged for a long period of time. Usually Fowler-Nordheim tunneling or hot-carrier injection mechanisms are used to modify the amount of charge stored on the floating gate.
In many IC processes, extra processing steps are required to produce FGMOSFETs along with MOSFETs. These extra steps increase the cost of making integrated circuits that contain FGMOSFETs and MOSFETs. A process that reduces the number of steps to manufacture FGMOSFETs along with MOSFETs is desirable.